Alphaviruses comprise a set of genetically, structurally, and serologically related arthropod-borne viruses of the Togaviridae family. Twenty-six known viruses and virus subtypes have been classified within the alphavirus genus, including, Sindbis virus, Semliki Forest virus, Ross River virus, and Venezuelan equine encephalitis virus.
Sindbis virus is the prototype member of the Alphavirus genus of the Togaviridae family. Its replication strategy has been well characterized in a variety of cultured cells and serves as a model for other alphaviruses. Briefly, the genome of Sindbis (like other alphaviruses) is an approximately 12 kb single-stranded positive-sense RNA molecule which is capped, polyadenylated, and contained within a virus-encoded capsid protein shell. The nucleocapsid is further surrounded by a host-derived lipid envelope, into which two viral-specific glycoproteins, E1 and E2, are inserted and anchored by a cytoplasmic tail to the nucleocapsid. Certain alphaviruses (e.g., SFV) also maintain an additional protein, E3, which is a cleavage product of the E2 precursor protein, PE2.
After virus particle adsorption to target cells, penetration, and uncoating of the nucleocapsid to release viral genomic RNA into the cytoplasm, the replicative process occurs via four alphaviral nonstructural proteins (nsPs), translated from the 5′ two-thirds of the viral genome. Synthesis of a full-length negative strand RNA, in turn, provides template for the synthesis of additional positive strand genomic RNA and an abundantly expressed 26S subgenomic RNA, initiated internally at the junction region promoter. The alphavirus structural proteins are translated from the subgenomic 26S RNA, which represents the 3′ one-third of the genome, and like the nsPs, are processed post-translationally into the individual proteins.
Several members of the alphavirus genus are being developed as “replicon” expression vectors for use as vaccines and therapeutics. Replicon vectors may be utilized in several formats, including DNA, RNA, and recombinant replicon particles. Such replicon vectors have been derived from alphaviruses that include, for example, Sindbis virus (Xiong et al. (1989) Science 243:1188-1191; Dubensky et al., (1996) J. Virol. 70:508-519; Hariharan et al. (1998) J. Virol. 72:950-958; Polo et al. (1999) PNAS 96:4598-4603), Semliki Forest virus (Liljestrom (1991) Bio/Technology 9:1356-1361; Berglund et al. (1998) Nat. Biotech. 16:562-565), and Venezuelan equine encephalitis virus (Pushko et al. (1997) Virology 239:389-401). A wide body of literature has now demonstrated efficacy of alphavirus replicon vectors for applications such as vaccines (see for example, Dubensky et al., ibid; Berglund et al., ibid; Hariharan et al., ibid, Pushko et al., ibid; Polo et al., ibid; Davis et al. (2000) J Virol. 74:371-378; Schlesinger and Dubensky (1999) Curr Opin. Biotechnol. 10:434-439; Berglund et al. (1999) Vaccine 17:497-507). Generally, speaking, a “replicon” particle refers to a virus particle containing a self-replicating nucleic acid. The replicon particle itself is generally considered replication incompetent or “defective,” that is no progeny replicon particles will result when a cell is infected with a replicon particle. Through the years, several synonymous terms have emerged that are used to describe replicon particles. These terms include recombinant viral particle, recombinant alphavirus particle, alphavirus replicon particle and replicon particle. However, as used herein, these terms all refer to a virion-like unit containing a virus-derived RNA vector replicon, specifically, an alphavirus RNA vector replicon. Moreover, these terms may be referred to collectively as vectors, vector constructs or gene delivery vectors.
Currently, several alphaviruses are being developed as gene delivery systems for vaccine and other therapeutic applications. Although generally quite similar in overall characteristics (e.g., structure, replication), individual alphaviruses may exhibit some particular property (e.g., receptor binding, interferon sensitivity, and disease profile) that is unique. To exploit the most desirable properties from each virus a chimeric replicon particle approach has been developed. Specifically, a chimeric alphavirus replicon particle may have RNA derived from one virus and one or more structural components derived from a different virus. The viral components are generally derived from closely related viruses; however, chimeric virus particles made from divergent virus families are possible.
It was previously demonstrated that chimeric alphavirus replicon particles can be generated, wherein the RNA vector is derived from a first alphavirus and the structural “coat” proteins (e.g., envelope glycoproteins) are derived from a second alphavirus (see, for example U.S. patent application Ser. No. 09/236,140; see also, U.S. Pat. Nos. 5,789,245, 5,842,723, and 6,015,694; as well as WO 95/07994, WO 97/38087 and WO 99/18226). However, although previously-described strategies were successful for making several alphavirus chimeras, such chimeric particles are not always produced in commercially viable yields, perhaps due to less efficient interactions between the viral RNA and structural proteins, resulting in decreased productivity.
Thus, there remains a need for compositions comprising and methods of making and using chimeric replicon particles and replicons, for example for use as gene delivery vehicles having altered cell and tissue tropism and/or structural protein surface antigenicity.